The present invention relates to a cylindrical filter element and to a method for producing such a cylindrical filter element.
Cylindrical filter elements having film end plates as a closure are known, for example, from Duerrstein et al., U.S. Pat. No. 5,736,040 (=DE 44 28 139). The end plates on the end faces of the filter element have an elastic design and are made of films which are heated for joining to the filter medium. The heating causes the films to expand toward the filter medium and partially penetrate same, thereby forming a seal on the end face of the filter folds. At the same time, the film material is cured by the heating, thus forming a firm connection by adhesion to the filter material.
A filter cartridge is known from Coulonvaux, U.S. Pat. No. 5,547,480 (=WO 95/19832) which is arranged in a housing having an inlet for untreated air and an outlet for clean air. Such a filter cleans the intake air in an internal combustion engine. The filter cartridge is braced between two concentric surfaces of the filter housing. The bracing action simultaneously creates a seal, which must be reliably maintained during shock or vibration.
German Patent Application No. DE 199 30 614 A1 (=U.S. Ser. No. 10/032,504) describes a filter cartridge, in particular for the air filter of an internal combustion engine. The filter cartridge is made of a filter paper which is folded in a zigzag shape and annularly joined to a cylindrical casing of the filter element. The end faces of the filter medium are cast inside a base plate and a cover plate. In the cylindrical interior of the filter element is situated a support tube having an elastic zone at its outlet end which is formed by axial slits in the end of the tube. The end of the support tube is made expandable in the radial direction by means of the elastic zone. Since the support tube is cast inside the cover plate, it expands along with the cover plate onto the connecting sleeve during installation of the filter element, thereby maintaining a positive-fit connection between the support tube and the cover plate.
Finally, a cylindrical filter element is described in German Patent Application No. DE 198 29 989 A1 which has resilient end plates which terminate the end faces of a filter medium. A support tube has elastic zones on its ends, formed by slits, which have a positive fit with the resilient end plates. This connection yields when sealing surfaces are mounted on corresponding connection pieces in the filter housing, so that an additional seal is not necessary. Integration of the support tube into the filter element facilitates installation in the housing.
The film end plates of the known filter elements are usually adhered so as to be even with the support tube. This often results in poor adhesion quality due to the production process in which the film is subjected, for example, to elevated temperatures (approximately 220° C.) for an extended period and fails to cure completely. This can lead to swelling of the end plates, and the adhesive bond can partially detach or pull away under certain conditions. This is especially disadvantageous if the filter is used as a fuel filter.